In an era dominated by yardage-gobbling offenses, the four teams playing in today's National Football League wild-card games are anomalies.

Although each has had its moments on offense this year, they have survived this far mainly because they all have defenses that would make Ronald Reagan proud.

Host Philadelphia and the New York Giants, the National Football Conference opponents (WDVM-TV-9, 3:30 p.m.), rank one-three in the league in fewest points surrendered.

Buffalo and host New York Jets, the American Football Conference foes (WRC-TV-4, noon), rate one-three in the conference in least yardage surrendered.

The Eagles have the kind of veteran, error-free defense that most championship teams want to emulate. Only Denver had better defensive statistics in the league this season.

The Giants' dramatic improvement since last year (4-12 to 9-7) is rivaled only by the rise of its defense, which allowed the second-most points in the NFL in 1980. No one in the league has better linebackers, especially since the arrival of rookie Lawrence Taylor, a future superstar.

Buffalo struggled in midseason when its defense stopped creating turnovers and when linebacker Shane Nelson, one-third of the Bermuda Triangle, was hurt. Nelson is on injured reserve, but the defense played much better the last three weeks.

The Jets, of course, have the New York Sack Exchange, that wondrous front four that recorded 66 sacks, one fewer than Oakland's league record. New York twice has registered nine sacks in a game, a month's work for many teams.

There is more bitterness in the Giant-Eagle game, where the tackling should be fearsome, and more chance of a one-sided outcome in the Bill-Jet contest, where New York has the most explosive and consistent offense of the four wild-card teams.

When the Giants beat Philadelphia, 20-10, in Veterans Stadium a month ago, the Eagles thought New York intentionally worked over halfback Wilbert Montgomery, giving him a series of borderline late hits.

Montgomery agreed. "I know that they play dirty ball," Montgomery said last week. "And when that happens, you look toward your teammates to help you out. The Giants were after me that last game, and maybe my teammates didn't know what was going on.

"But I heard them (the Giants) smartin' off, trying to intimidate me, trying to get me out of my game."

Although Montgomery gained 102 yards that afternoon, he had to be helped off the field three times.

"We're not a team that goes in much for dirty ball," said Montgomery, whose quotes were displayed prominently by New York newspapers. "But that's the way the Giants play every team and the officials should be aware of it. You don't try to stand someone up so someone else can come along and hit them. If you've got a clean hit, then hit 'em. But don't try to stand them up for the gang tackle."

Sparked by the aggressiveness of Taylor, the former North Carolina star who has nine sacks blitzing from his outside linebacker position, the Giants have not given up more than 17 points to any team since losing in overtime to Washington, 30-27, Nov. 15.

Quarterback Scott Brunner has started since Phil Simms hurt his shoulder in the Redskin game. The Giants are 4-1 with Brunner in charge, but Simms should be available today for the first time as a backup.

Coach Dick Vermeil says his team got its offense back in shape only after returning to the basic, go-right-at-them approach he first learned from George Allen.

Quarterback Ron Jaworski, sure to be booed heavily by the home fans if he gets off to a slow start, would benefit greatly if Montgomery can pound out the second 100-yard day against the Giants this season.

The Eagles could get a lift from veteran defensive end Claude Humphrey, who was removed from the injured-reserve list yesterday. To make room for Humphrey on the 45-man roster, Philadelphia placed rookie tight end Steve Folson on the injured-reserve list with a hamstring problem.

The Jet-Buffalo game should be more entertaining than the Eagle-Giant affair, if for no other reason than the quarterback-trapping antics of the Sack Exchange. End Mark Gastineau (20 sacks) puts on a show after each sack, twisting and turning his 6-5, 265-pound frame in all manner of directions. But fellow end Joe Klecko (20 1/2 sacks), a reformed truck driver turned bit actor, is a more rounded defender for now.

What is intriguing about this game is the matchup between Klecko and Co. and the Buffalo offensive line, which allowed the fewest sacks in the league (16).

The rivals split two contests, Buffalo winning the season opener, 31-0, and the fast-finishing Jets romping, 33-14, in October. New York has played slightly better lately, winning 10 of its last 13 while Buffalo captured four of its last five.

Under the guidance of quarterback Richard Todd, the Jet offense has matured weekly and now averages 337 yards a game. Todd enjoyed his best season as a pro working Offensive Coordinator Joe Walton's ball-control offense. But the Jets have become more explosive recently since receiver Lam Jones has switched from shuttling in plays to full-time duty.

Buffalo also can be productive, especially if halfback Joe Cribbs and receiver Jerry Butler are not checked properly. Quarterback Joe Ferguson, another of an aging group of AFC quarterbacks, didn't have his best season, but as he showed against San Diego last season, he is capable of heroic playoff performances.